The invention relates generally to magnetoresistive (MR) sensor materials and, particularly, to a thin film sensor which has a lanthanum-manganite (La--MnO.sub.3) based magnetoresistive element and which generates highly uniform gap magnetic fields for biasing the element.
In general, "magnetoresistance" refers to a resistance change produced in a magnetic sample when it is subjected to a magnetic field. This phenomenon typically occurs in magnetic materials in which the electrical resistance of the material is dependent upon the direction of magnetization in the material relative to the direction of current flow. One such material is a magnetically soft alloy of metal known as permalloy (e.g., approximately 80 at. % Ni and 20 at. % Fe). Permalloy and like materials are often referred to as low field MR materials.
In low field MR materials, the resistance changes as a function of the magnetization and current directions in the material according to: EQU .rho.(H)=.rho.(0)+.DELTA..rho..sub.max .multidot.cos.sup.2 (.theta.)
where .theta. is the angle between the direction of magnetization and the direction of current flow in the material; .rho.(0) is the isotropic resistivity; and .DELTA..rho..sub.max is the change in .rho. for the angle between the magnetization and current directions being changed from 0.degree. to 90.degree. (i.e., .DELTA..rho..sub.max is the maximum magnetoresistivity). Typically, .DELTA..rho..sub.max is only a few percent of .rho..
In general, the anisotropy field, which is defined as the field required to change the direction of magnetization by 90.degree., is fairly small at relatively low field strengths (e.g., only several to approximately 100 oersted (Oe)). Anisotropy is the directional dependence of magnetic properties, leading to the existence of easy or preferred directions of magnetization. In other words, magnetic materials have a better magnetic characteristic along one axis than along any other. The changes in resistivity are nearly linear for .theta..apprxeq.45.degree.. Often, some combination of longitudinal and transverse biasing is used to offset the direction of the magnetization from that of the current direction to cause the magnetoresistive effect. One common biasing mode is a barber pole arrangement of conductors on the magnetoresistive surface to skew the current direction between these conductors away from the magnetoresistive longitudinal direction.
Magnetoresistive sensors use the characteristics of certain MR materials to detect magnetic field changes. One use for magnetoresistive sensors is reading information from magnetic recordings. Such a sensor, often referred to as a magnetoresistive read head, has current flowing in one end and out another. When the recording medium, which has a changing magnetization pattern due to the information stored on it, moves relative to the read head, the stray fields from the medium cause a change in the direction of magnetization. Thus, the resistance of the head changes. By using a constant current source to drive the head, the change in resistance can be measured by determining the change in voltage across the read head's terminals.
A disadvantage with low field magnetoresistive sensors is that the relative signal levels are usually small. In other words, sensitive circuits (e.g., bridge type circuits) are needed to detect the changes in resistance.
In contrast to low field MR materials, high field MR materials (e.g., La-manganite) exhibit little or no change in resistance due to directional changes in magnetic field. Rather, the resistance of a high field MR material changes upon application of a magnetic field primarily as a function of the applied field magnitude. The change occurs as the material essentially undergoes a metal-insulator transition when subjected to the magnetic field. Due to this transition, the changes in resistance levels are a greater percentage of the overall resistance in high field MR materials. Therefore, sensors using high field MR materials permit less complex electronics for detecting and processing resistance changes.
The use of high field MR materials as magnetoresistive sensors, however, differs from the traditional uses of MR materials in several respects. For example, since the magnetoresistance arises from a metal-insulator transition in La-manganite, the resistance change is very large but only occurs over a relatively narrow temperature range which defines the transition. Another difference is that there is no directional dependence of the resistance with respect to the current flow and magnetic field direction. And lastly, the magnetic fields required to bias the material for high magnetoresistive sensitivity are relatively large and much greater than those required in traditional magnetoresistive sensing elements. These differences present practical problems in implementing the sensors.
Therefore, a uniform biasing field provided by a small scale geometry is desired for practical operating reasons. Further, it is desired that such geometry be fabricated by film deposition methods and permit temperature isolation of the magnetoresistive element.
As stated above, presently available magnetoresistive sensors detect the presence of an externally applied magnetic field. In other words, such sensors detect the presence of a magnetized material such as a magnetic recording medium. For this reason, such sensors are not suited for a number of uses other than magnetic recording read heads such as position and/or rotation sensing, bar or dot code reading. Therefore, a magnetoresistive sensor is desired for detecting the presence of magnetized or magnetizable material, as opposed to magnetized material only.